The researchers say that the differences may help explain why C-section babies have a higher risk of asthma, diabetes, and obesity. The study showed that babies born by C-section had fewer colonies of Escherichia and Shigella bacteria, two types of bacteria believed to be important in a developing immune system.

“We are still learning about this, but it seems that there is an order [to the colonization],” Kozyrskyj told Time. “And if there is an order, then timing is important.”

Formula-fed babies had more bacteria of a certain type than breastfed babies, which concerned the researchers because the presence of one, Clostridium difficile, which is linked with diarrhea in adults, might push out the same immune system bugs lacking in the C-section babes.

“The next step is to link these changes to childhood conditions and assess whether these children have different risks for diseases, how severe their conditions are and what the patterns of these diseases are,” Kozyrskyj told Time.